This invention relates to optical fiber technology and optical fiber dispensers, and, more particularly, to an optical fiber dispenser structured to minimize the adverse consequences of thermal expansion differences between the optical fiber pack and the bobbin.
Optical fibers are strands of glass fiber processed so that light transmitted therethrough is subject to total internal reflection. Glass optical fibers are typically fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage. As an example of the dimensions, in a typical configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the optical fiber plus the polymer buffer (sometimes termed an "optical fiber cable") is about 250 micrometers (approximately 0.010 inches).
For such very fine optical fibers, the handling of the optical fiber cable to avoid damage that might reduce its mechanical strength and/or light transmission properties becomes an important consideration. In one approach, the optical fiber cable is wound onto a cylindrical or tapered cylindrical bobbin (collectively termed herein a "tapered" cylindrical bobbin, even though the angle of the taper may be zero) with many turns adjacent to each other in a side-by-side fashion. After one layer is complete, another layer of optical fiber cable is wound on top of the first layer, and so on. A weak adhesive is typically applied to the layers of optical fiber cable, to hold them in place. The final assembly of the bobbin and the wound layers of optical fiber cable is termed a dispenser, and the mass of wound optical fiber cable is termed the fiber pack. When the optical fiber cable is later to be used, the optical fiber cable is paid out from the dispenser in a direction generally parallel to the axis of the tapered cylinder.
In general, the dispenser is prepared at one temperature, but subjected to other temperatures during storage, handling, and service. The bobbin and the optical fiber pack have different thermal expansion coefficients, and the resulting thermally induced strains and stresses can cause various defects in the dispenser as a result of temperature changes. The effective thermal expansion coefficient of the optical fiber pack is low in the circumferential direction, about the same as that of the glass in the optical fiber. The effective thermal expansion coefficient of the optical fiber pack is much higher in the longitudinal direction (parallel to the cylinder axis), about the same as that of the polymeric buffer material. The thermal expansion coefficient of the bobbin cannot be readily matched to this anisotropic state, so that thermal expansion mismatch in at least one direction is virtually inevitable.
A number of approaches have been proposed for avoiding or at least reducing the magnitude of the thermal expansion stresses in the dispenser. Such a reduction in thermally induced stresses can improve the life expectancy and the associated performance of the optical fiber dispenser by reducing the incidence of defects in the optical fiber pack. For various reasons, these approaches have not proved entirely successful. Accordingly, there remains a need for an improved approach to the construction of optical fiber dispensers to minimize the adverse effects of thermal expansion mismatches. The present invention fulfills this need, and further provides related advantages.